Contrail avoidance and its role in aviation climate impact mitigation

The aviation sector faces mounting pressure to reduce its climate impact, with ambitious targets set nationally and internationally. The UK’s 2021 Sixth Carbon Budget, for example, aims to cut national emissions (including those from international aviation and shipping) by 78% by 2035 (UK Government, 2021). Similarly, the International Air Transport Association (IATA) pledged in October 2021 to achieve global net-zero carbon emissions by 2050 (IATA, 2021). Beyond CO2 emissions reductions, aviation must also address non-CO2 effects, which are currently estimated to account for around 66% of the aviation effective radiative forcing (ERF) (Lee et al., 2021).

Across the several net-zero emission pathways proposed, a key part involves a transition to alternative fuels, such as sustainable aviation fuel (SAF) and hydrogen, together with targeted strategies for non-CO2 effects mitigation (Dray et al., 2022), including through contrail avoidance (Mannstein 2005; Teoh et al. 2020). This consists in rerouting aircraft to minimise the formation of persistent contrails, the thin line-shaped clouds that form behind an aircraft under liquid water saturation conditions due to mixing between the warm and moist exhaust and the cool ambient air. When the ambient relative humidity exceeds ice saturation, contrails can persist and spread, leading to the formation of contrail cirrus, responsible for the largest aviation warming effect on climate.

Contrail avoidance relies on integrating real-time weather data into flight planning in order to avoid the ice supersaturated regions (ISSRs) where contrails are likely to form and persist. A recent meta-analysis of contrail avoidance modelling studies suggested that half of the contrail length could be avoided at a penalty of 1% increase in fuel burn (Dray et al., 2022). In 2021, the first-ever operational contrail avoidance trial which took place in the Maastricht Upper Area Control (MUAC) region, concluded that contrail avoidance could be an efficient method for mitigating the climate impact of aviation, with persistent contrails potentially avoided by flying up to 2000 ft lower or higher (Sausen et al., 2023). In another trial in the US, Google Research together with American Airlines used Artificial Intelligence based predictions, combined with Breakthrough Energy’s pycontrails open-source contrail model to avoid contrail formation during 70 test flights over 6 months. Their post flight analysis using satellite images found a 54% reduction in contrail cover, with an associated fuel burn penalty of 2%. However, these first trials also highlighted a series of important open questions that need to be addressed before any large-scale contrail avoidance operational implementation. In particular, the main concern remains the current poor skill of forecast models to predict persistent contrails (Sausen et al., 2023; Hofer et al., 2024), together with the potential negative impact of the additional emissions caused by the flight rerouting. 

The aim of this project is to investigate the climatic effect of contrail avoidance strategies for current and future generation aircraft. The modelling approach will involve the use and development of the state-of-the-art UK Met Office Unified Model, including its recent contrail cirrus parameterisations and weather forecast models. While relatively flexible to allow for your interests, the project is likely to involve:

·      Assessing the UM model skill to forecast ice supersaturated regions, using a series of in-situ observations (i.e. IAGOS) and reanalysis products;

·      Quantifying the reduction in contrail cirrus ERF for different contrail avoidance strategies designed for current and future generation aircraft;

·      Quantifying the total aviation ERF for different contrail avoidance strategies for current and future generation aircraft;

·      Exploring the role of different climate metrics for setting aviation climate mitigation targets.